It's probably not too dressy, but if it's shorter than floor length, that's probably better. We did the Disney 3-day a couple years ago and we saw everything from "Sunday best" to long formals (very few of the latter) on formal night. Being a family-oriented cruise, it seemed a tiny bit less dressy than some other cruise lines. On the other hand, no one will look at you funny if you look like a princess!
Have a wonderful cruise!